Low Altitude Solar Magnetic Reconnection, Type III Solar Radio Bursts, and X-ray Emissions.

Type III solar radio bursts are the Sun's most intense and frequent nonthermal radio emissions. They involve two critical problems in astrophysics, plasma physics, and space physics: how collective processes produce nonthermal radiation and how magnetic reconnection occurs and changes magnetic energy into kinetic energy. Here magnetic reconnection events are identified definitively in Solar Dynamics Observatory UV-EUV data, with strong upward and downward pairs of jets, current sheets, and cusp-like geometries on top of time-varying magnetic loops, and strong outflows along pairs of open magnetic field lines. Type III bursts imaged by the Murchison Widefield Array and detected by the Learmonth radiospectrograph and STEREO B spacecraft are demonstrated to be in very good temporal and spatial coincidence with specific reconnection events and with bursts of X-rays detected by the RHESSI spacecraft. The reconnection sites are low, near heights of 5-10 Mm. These images and event timings provide the long-desired direct evidence that semi-relativistic electrons energized in magnetic reconnection regions produce type III radio bursts. Not all the observed reconnection events produce X-ray events or coronal or interplanetary type III bursts; thus different special conditions exist for electrons leaving reconnection regions to produce observable radio, EUV, UV, and X-ray bursts.

Mobilization of peripheral blood stem cells with docetaxel and cyclophosphamide (CY) in patients with metastatic breast cancer: a randomized trial of 3 vs 4 g/m2 of CY.

The purpose of this study was to develop a regimen of docetaxel, cyclophosphamide (CY) and filgrastim for mobilization of peripheral blood stem cells (PBSC) in patients with metastatic breast cancer (n = 66). A phase I trial of CY 2, 3 or 4 g/m2 with docetaxel 100 mg/m2, in consecutive cohorts of four patients each, did not reveal any dose-limiting toxicities and subsequent patients were randomized to receive 3 or 4 g/m2 of CY. The median yield of CD34+ cells from all patients was 11.06x10(6)/kg (range, 0.03-84.77) from a median of two aphereses (range, 1-7); 6.52x10(6) CD34+ cells/kg/apheresis (range, 0.01-52.07). Target CD34+ cell doses > or =2.5 and > or =5.0x10(6)/kg were achieved in 89% and 79%, respectively. There were no statistically significant differences in CD34+ cell yields or target CD34+ cell doses achieved following 3 or 4 g/m2 of CY. Patients with only one prior chemotherapy regimen yielded a median of 12.82x10(6) CD34+ cells/kg/apheresis compared to 5.85 for those receiving > or =2 regimens (P = 0.03). It was concluded that the combination of docetaxel, 100 mg/m2, CY 3 g/m2 without mesna could be administered with acceptable toxicity with collection of adequate quantities of PBSC from the majority of patients.

Second attempts at mobilization of peripheral blood stem cells in patients with initial low CD34+ cell yields.

The purpose of this study was to determine the effectiveness of second mobilization strategies in patients who yielded < 2.5 x 10(6) CD34+ PBSC/kg after initial mobilization. Repeat mobilization attempts were made with chemotherapy and G-CSF (n = 61) or G-CSF alone (n = 58) in patients who failed initial mobilization with chemotherapy and G-CSF (n = 92) or G-CSF alone (n = 27). A median of 0.27 x 10(6) CD34+ cells/kg per apheresis was collected after the second mobilization, compared with 0.16 with initial harvests (p = 0.0001). Forty-eight percent achieved a target CD34+ cell dose > or = 2.5 x 10(6)/kg when harvests from the first and second mobilizations were combined. Fifteen of 17 patients (88%) with > or = 1.5 x 10(6) CD34+ cells/kg harvested after first mobilization had > or = 2.5 x 10(6) CD34+ cells/kg collected when first and second harvests were combined, as compared with 42 of 102 (41%) achieving < 1.5 x 10(6) CD34+ cells/kg with first PBSC harvests (p = 0.0001). Second mobilizations with chemotherapy and G-CSF or G-CSF alone resulted in similar CD34+ cell yields. Toxicities of second mobilizations were comparable with those of first mobilizations. Seventy-nine patients (66%) received high-dose chemotherapy with PBSC support, with recovery of neutrophils and platelets in a median of 11 and 15 days, respectively. Transplant-related mortality was 4%, and event-free survival at 2 years was 0.34. It was concluded that second mobilization attempts in patients who fail to achieve > or = 2.5 x 10(6) CD34+ cells/kg on initial mobilization were successful in 48% of patients. G-CSF alone was as effective as chemotherapy plus G-CSF in mobilizing CD34+ cells and was associated with less morbidity.

Collection of peripheral blood progenitor cells after the administration of cyclophosphamide, etoposide, and granulocyte-colony-stimulating factor: an analysis of 497 patients.

BACKGROUND: There is great interpatient variability in the number of peripheral blood stem cells collected, as measured by CD34+ cell content, after the administration of chemotherapy and a growth factor. The ability to predict patients who fail to yield adequate quantities of CD34+ cells would be of value. However, very few reports include large numbers of patients treated in an identical fashion. STUDY DESIGN AND METHODS: Between 1991 and 1995, 497 consecutive patients with a variety of malignant diseases received cyclophosphamide (4 g/m2), etoposide (600 mg/m2), and granulocyte-colony-stimulating factor (6 micrograms/kg/day) for mobilization and collection of a target dose > or = 2.5 x 10(8) CD34+ cells per kg. Multivariate analyses were performed to determine the factors associated with failure to achieve this target harvest. RESULTS: A median of 14.71 x 10(6) CD34+ cells per kg (range, 0.08-137.55) was harvested with a median of 2 (range, 1-11) apheresis procedures. Ninety-one percent of patients yielded > or = 2.5 x 10(5) CD34+ cells per kg. Patients with Stage II-III breast cancer, who had pretreatment platelet counts > or = 150 x 10(9) per L and patients who underwent < or = 1 prior chemotherapy regimen had improved CD34+ cell yields. However, most patients with adverse risk factors yielded > or = 2.5 x 10(6) CD34+ cells per kg. CONCLUSION: A regimen of cyclophosphamide, etoposide, and granulocyte-colony-stimulating factor led to the successful collection of adequate numbers of CD34+ cells in most patients without excessive toxicity. These observations confirm previous reports that intense prior therapy adversely affects the quantity of CD34+ cells harvested. Pretreatment and posttreatment variables did not predict with any certainty the small fraction of patients who fail to yield > or = 2.5 x 10(6) CD34+ cells per kg via multiple apheresis procedures.

Characterization of a culture-derived CD15+CD11b- promyelocytic population from CD34+ peripheral blood cells.

Selected CD34+ cells from mobilized apheresis products were cultured in serum-free or serum-containing media supplemented with granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and stem cell factor (SCF; c-kit ligand). We examined the emergence of a CD15+CD11b- population, which appeared morphologically to be promyelocytes. This CD15+CD11b- population can be further expanded in culture into morphologically mature granulocytes. In an attempt to characterize this culture-derived CD15+CD11b- promyelocytic population, single cells were clone sorted into wells of a Terasaki plate containing various growth factors. We compared the growth factor requirements and kinetics of this apheresis culture-derived CD15+CD11b- population to the CD15+CD11b- population from fresh bone marrow samples. Our studies indicate that the CD15+CD11b- promyelocytic population from bone marrow and blood are equivalent in their ability to proliferate and in their requirements for growth factors. The CD15+CD11b- population in vitro shows a high proliferative capacity when compared with the other CD15/CD11b populations (CD15-CD11b-, CD15+CD11b+, CD15-CD11b+). Thus, we can manipulate CD34+ cells in vitro to proliferate and differentiate toward a mature neutrophil lineage. The CD15+CD11b- promyelocytic population derived from this culture may represent the most effective cultured cell population for therapeutic reduction of neutropenia in vivo based on both its stage of differentiation and its proliferative potential.

Engraftment and outcomes of patients receiving myeloablative therapy followed by autologous peripheral blood stem cells with a low CD34+ cell content.

Engraftment kinetics after high-dose chemotherapy (HDC) were evaluated in patients receiving autologous peripheral blood stem cell (PBSC) infusions with a low CD34+ cell content. Forty-eight patients were infused with < 2.5 x 10(6) CD34+ cells/kg; 36 because of poor harvests and 12 because they electively received only a fraction of their harvested cells. A median of 2.12 x 10(6) CD34+ cells/kg (range, 1.17-2.48) were infused following one of seven different HDC regimens. All patients achieved absolute neutrophil counts > or = 0.5 x 10(9)/l at a median of day 11 (range, 9-16). Forty-seven patients achieved platelet counts > or = 20 x 10(9)/l at a median of day 14 (range, 8-250). Nine of 47 (19%) had platelet recovery after day 21, 4/47 (9%) after day 100 and one died on day 240 without platelet recovery. Twenty-six patients (54%) died of progressive disease in 51-762 days; 22 (46%) are alive at a median of 450 days (range, 94-1844), 17 (35%) of whom are surviving disease-free at a median of 494 days (range, 55-1263). No patient died as a direct consequence of low blood cell counts. These data demonstrate that PBSC products containing 1.17-2.48 x 10(6) CD34+ cells/kg resulted in relatively prompt neutrophil recovery in all patients but approximately 10% had delayed platelet recovery.

Collection of peripheral blood stem cells following administration of paclitaxel, cyclophosphamide, and filgrastim in patients with breast and ovarian cancer.

PURPOSE: To determine the toxicities and efficacy of paclitaxel, cyclophosphamide (Cy), and recombinant human granulocyte-colony stimulating factor (filgrastim) administered for mobilization and collection of peripheral blood stem cells (PBSC) in patients with breast and ovarian cancer. METHODS: One hundred and forty-one patients with breast (n = 115) or ovarian cancer (n = 26) received paclitaxel 170 mg/m2 and Cy 2 gm/m2 (n = 42) or paclitaxel 200 mg/m2, Cy 3 gm/m2 (n = 99), and filgrastim (6 micrograms/kg/day) followed by collection of PBSC by apheresis. RESULTS: The 2 dose levels of paclitaxel and Cy tested were well tolerated. The median yield of CD34+ cells from all patients was 6.53 x 10(6)/kg (range, 0.11-51.76) collected with a median of 2 aphereses (range, 1-8). The target dose of 2.5 x 10(6) CD34+ cells/kg was achieved in 85% of patients. The mean daily collection of CD34+ cells was 5.46 x 10(6)/kg for patients receiving 200 mg/m2 of paclitaxel and 3 gm/m2 of Cy as compared to 2.77 for patients receiving the lower doses (p = 0.0005). Increasing the dose of paclitaxel and Cy did not significantly increase the fraction of patients achieving a target dose of 2.5 x 10(6) CD34+ cells/kg (87% vs 81%, p = 0.367) but did increase the fraction achieving a target of 5.0 x 10(6) CD34+ cells/kg (73% vs 45%, p = 0.002). The mean daily collection of CD34+ cells for patients who had received only 1 prior chemotherapy regimen was 6.59 x 10(6)/kg as compared to 3.47 for patients who had received more than 1 prior chemotherapy regimen (p < 0.0001). Prior radiation therapy (p = 0.003) and patient performance status (p = 0.047) were adverse risk factors for achieving a target dose of > or = 2.5 x 10(6) CD34+ cells/kg. CONCLUSIONS: The combination of paclitaxel, Cy, and filgrastim can be administered with acceptable toxicity, allowing collection of adequate quantities of PBSC from the majority of patients with breast and ovarian cancer. Increasing the doses of paclitaxel and Cy increased the number of CD34+ cells collected and decreased the number of apheresis procedures necessary to collect target cell doses. However, increasing drug doses did not increase the fraction of patients yielding the minimum CD34+ target dose of 2.5 x 10(6)/kg. Collection of PBSC early in the disease course is the best strategy to assure optimal CD34+ cell doses in all patients.

The relationship between parental communication deviance and relapse in schizophrenic patients in the 1-year period after hospital discharge. A pilot study.

The present pilot study examined the ability of parental communication deviance (CD) to predict relapse in schizophrenic patients in the 1-year period after hospital discharge. Measures of CD were obtained from verbatim transcripts of family problem-solving discussions for 20 schizophrenic patients and their parents at an initial assessment and before the patients' hospital discharge. Relapse was assessed using the Brief Psychiatric Rating Scale at 3-month intervals after discharge. Findings indicated that parental CD measured at an assessment immediately before the patient's release from the hospital was moderately correlated (r = .49, p < .05) with relapse in the 1-year follow-up period. Results are discussed with respect to the possible role of parental CD as an environmental stressor in the period after discharge. Potential contributors to parental CD, including genetically transmitted cognitive deficits and stress resulting from resuming the care-giver role, are also discussed.

High-dose chemotherapy and autologous peripheral blood progenitor cell transplant for the treatment of Hodgkin's disease.

This paper evaluates a comprehensive strategy of chemotherapy mobilization of peripheral blood progenitor cells (PBPCs) followed by high-dose chemotherapy for the treatment of refractory or relapsed Hodgkin's disease (HD). Patients with relapsed or refractory HD were enrolled to receive cyclophosphamide, etoposide +/- cisplatin (CE +/- P) and rhG-CSF mobilization of PBPCs. Patients achieving < or = 2.5 x 10(6) CD34+ cells/kg following initial mobilization were eligible to receive a second course of CE +/- P. Unmanipulated PBPCs alone were infused following administration of high-dose carmustine, etoposide, cytarabine arabinoside and cyclophosphamide (BEAC). Thirty-eight consecutive patients with relapsed or refractory HD were initially enrolled to receive CE +/- P. Analysis was performed on an intent-to-treat basis. A median of 6.4 x 10(6) CD34+ cells/kg (range 0.66-62.3) were collected with a median of 3 (range 2-9) leukaphereses. Twenty-eight of 38 (74%) patients achieved > or = 2.5 x 10(6) CD34+ cells/kg. Analysis of variables potentially effecting mobilization of CD34+ cells revealed that only the amount of prior chemotherapy statistically influenced collecting CD34+ cells (P = 0.005). Two of six patients undergoing a second mobilization procedure achieved > or = 2.5 x 10(6) CD34+ cells/kg for a total of 30 patients eligible to proceed with high-dose BEAC. The 3-year Kaplan-Meier estimate of overall survival (OS) and progression-free survival (PFS) for all 38 patients is 65 and 53%, respectively. The 3-year OS and PFS for the 28 patients receiving BEAC is 77 and 64% respectively vs 33 and 30% for the 10 patients not receiving BEAC. The strategy of CE +/- P and BEAC was well tolerated with a 100-day treatment-related mortality of 3.6%. All patients experienced rapid and sustained hematologic recovery with PBPCs alone. The median time to an ANC > or = 5 x 10(9)/1 and platelet transfusion independence was 10 days. Although development of better strategies to mobilize PBPCs may benefit additional patients, currently the best strategy to collect PBPCs is early before patients have received extensive chemotherapy treatment. Collection of PBPCs immediately following initial relapse or induction failure using CE +/- P for PBPC mobilization allows sufficient PBPCs to be collected in greater than 90% of patients. Treatment of refractory or relapsed HD utilizing a strategy of CE +/- P PBPC mobilization for hematopoietic reconstitution following high-dose BEAC is associated with acceptable toxicity and rapid engraftment. A 3-year PFS greater than 60% can be achieved in the community hospital setting.

Immunocytochemical analysis of tumor cells in pre- and post-culture peripheral blood progenitor cell collections from breast cancer patients.

We examined peripheral blood progenitor cell (PBPC) collections and CD(34+)-selected fractions cultured in PIXY321, a fusion protein comprising analog interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) domains, for the presence of contaminating tumor cells from 14 patients with advanced-stage breast cancer. Five of the 14 (36%) pre-culture PBPC specimens contained immunocyto-chemically (ICC)-detectable tumor cells using two different cocktails of monoclonal antibodies (mAbs). After 10 days in culture with PIXY321, the CD(34+)-selected fractions showed a median 23.6-fold expansion of hematopoietic cells. No ICC-positive tumor cells were detected in any post-culture specimens. We conclude that in vitro expansion of CD(34+)-selected PBPCs with PIXY321 can expand hematopoietic cell populations apparently without risk of expanding contaminating breast cancer cell populations.

An analysis of engraftment kinetics as a function of the CD34 content of peripheral blood progenitor cell collections in 692 patients after the administration of myeloablative chemotherapy.

The CD34 antigen is expressed by committed and uncommitted hematopoietic progenitor cells and is increasingly used to assess stem cell content of peripheral blood progenitor cell (PBPC) collections. Quantitative CD34 expression in PBPC collections has been suggested to correlate with engraftment kinetics of PBPCs infused after myeloablative therapy. We analyzed the engraftment kinetics as a function of CD34 content in 692 patients treated with high-dose chemotherapy (HDC). Patients had PBPCs collected after cyclophosphamide based mobilization chemotherapy with or without recombinant human granulocyte colony-stimulating factor (rhG-CSF) until > or = 2.5 x 10(6) CD34+ cells/kg were harvested. Measurement of the CD34 content of PBPC collections was performed daily by a central reference laboratory using a single technique of CD34 analysis. Forty-five patients required a second mobilization procedure to achieve > or = 2.5 x 10(6) CD34+ cells/kg and 15 patients with less than 2.5 x 10(6) CD34+ cells/kg available for infusion received HDC. A median of 9.94 x 10(6) CD34+ cells/kg (range, 0.5 to 112.6 x 10(6) CD34+ cells/kg) contained in the PBPC collections was subsequently infused into patients after the administration of HDC. Engraftment was rapid with patients requiring a median of 9 days (range, 5 to 38 days) to achieve a neutrophil count of 0.5 x 10(9)/L and a median of 9 days (range, 4 to 53+ days) to achieve a platelet count of > or = 20 x 10(9)/L. A clear dose-response relationship was evident between the number of CD34+ cells per kilogram infused between the number of CD34+ cells per kilogram infused and neutrophil and platelet engraftment kinetics. Factors potentially influencing the engraftment kinetics of neutrophil and platelet recovery were examined using a Cox regression model. The single most powerful mediator of both platelet (P = .0001) and neutrophil (P = .0001) recovery was the CD34 content of the PBPC product. Administration of a post-PBPC infusion myeloid growth factor was also highly correlated with neutrophil recovery (P = .0001). Patients receiving high-dose cyclophosphamide, thiotepa, and carboplatin had more rapid platelet recovery than patients receiving other regimens (P = .006), and patients requiring 2 mobilization procedures versus 1 mobilization procedure to achieve > or = 2.5 x 10(6) CD34+ cells/kg experienced slower platelet recovery (P = .005). Although a minimal threshold CD34 dose could not be defined, > or = 5.0 x 10(6) CD34+ cells/kg appears to be optimal for ensuring rapid neutrophil and platelet recovery.

Neurobehavioral characteristics of CGG amplification status in fragile X females.

Neurobehavioral correlates of CGG amplification were studied in 17 nonretarded adult female carriers of fragile X syndrome. The results revealed a significant relationship between IQ and the number of CGG repeats in the 5' untranslated region of the FMR1 gene. Women with a full mutation (> 200 CGG repeats) scored below average in IQ, visual-spatial perception, visual-spatial organization, and executive function. There were no differences in fine motor dexterity or memory as a function of CGG amplification status. A history of major depressive disorder was identified in 71% of the sample, but incidence of depression was not associated with the degree of CGG amplification. Schizotypal features were noted in 18%. No intellectual or neuropsychological deficit was found in women with a premutation (< 200 CGG repeats). Decrements in IQ, visual-spatial perception, and executive function appear to arise as a consequence of the CGG amplification.

Rapid and sustained hematopoietic reconstitution by peripheral blood stem cell infusion alone following high-dose chemotherapy.

We utilized mobilized peripheral blood stem cells (PBSC) as sole support for hematologic reconstitution following high-dose chemotherapy in 52 patients with advanced solid tumors and lymphoma. PBSC were collected by large scale leukapheresis after mobilization with chemotherapy. Each apheresis product was analysed for total nucleated cells, CFU-GM and CD34+ content. Disease-specific high-dose chemotherapy regimens were administered followed by thawed PBSC. Colony-stimulating factors were not administered. The median time to an absolute neutrophil count > 0.5 x 10(9)/l was 13 days (range 9-26 days) and median time to a sustained platelet count > 20 x 10(9)/l without transfusion support was 10 days (range 5-43 + days). There was no difference in time to recovery by dose-intensive regimen or underlying disease. The times to recover ANC and platelets both correlated significantly with increasing doses of PBSC as assayed by CD34+ cells and CFU-GM. All four patients with prolonged platelet recovery times received < 20 x 10(4) CFU-GM/kg, establishing this as a threshold value for PBSC infusion. There were no late transient or sustained graft failures. For 26 patients alive 1 year after infusion, the mean total leukocyte count is 6.3 x 10(9)/l, mean hematocrit 35.5% and mean platelet count 182 x 10(9)/l. Thirteen patients followed at least 24 months after PBSC infusion have essentially normal blood counts. Mobilized peripheral blood progenitors are an effective source of stem cells which afford rapid and complete hematopoietic engraftment after myelo-suppressive chemotherapy regimens. Engraftment appears sustained with no late failures.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral blood stem cell mobilization by chemotherapy with and without recombinant human granulocyte colony-stimulating factor.

Chemotherapy can serve as a stimulus for mobilizing hematopoietic progenitor cells to the peripheral blood for harvest via leukapheresis. Mobilized peripheral blood stem cells (PBSC) support rapid hematologic reconstitution after bone marrow aplasia induced by intensive myelosuppressive treatments. Our purpose was to develop effective mobilization regimens allowing collection of large quantities of PBSC. We administered high-dose cyclophosphamide (HDC, 4 gm/m2) or cyclophosphamide (4 gm/m2) plus etoposide (600 mg/m2) (HDCE) in a nonrandomized, sequential fashion to 94 patients with breast cancer, lymphoma, and other malignancies with collection of PBSC via leukapheresis during white blood cell (WBC) recovery from nadir counts. Each apheresis product was analyzed for total nucleated cell number, granulocyte-macrophage colony-forming units (CFU-GM) and CD34+ cells. Twenty-four additional patients with comparable pretreatment characteristics received HDCE plus recombinant human granulocyte colony-stimulating factor (HDCE+G) after chemotherapy through the end of apheresis. Patients receiving HDC were matched for age, sex, and disease but were more heavily pretreated. HDCE was superior to HDC in mean daily CFU-GM and CD34+ yield (p < 0.05), even when groups were adjusted for performance status and amount of prior therapy. HDCE+G led to 3.7 times more CFU-GM and 4.7 times more CD34+ cells than HDCE. Target PBSC yield, defined as > 20 x 10(4) CFU-GM/kg and >4 x 10(8) cells/kg, was achieved by 92% of HDCE+G patients after a median of three aphereses, 56% of HDCE patients after five aphereses, and 16% of HDC patients after six apheresis (p < 0.0001). Prior chemotherapy inversely correlated with the quantity of PBSC harvested regardless of regimen utilized. Our results demonstrate effective chemotherapy regimens for harvesting hematopoietic progenitors in a diverse patient population. HDCE+G produced the highest number of progenitors, suggesting that increasing dose intensity and adding rhG-CSF enhances mobilization. Correlation between cumulative CD34+ and CFU-GM allows real-time flow cytometric analysis of the number of aphereses required to harvest target numbers of PBSC.

Morphological and molecular characterization of spontaneous myogenic differentiation in a human rhabdomyosarcoma cell line.

Skeletal muscle differentiation consists of an ordered withdrawal of committed cells from the cell cycle and their fusion to form multinucleated myotubes. To determine if differentiation of malignant myoblasts parallels that of normal skeletal muscle, a cell line (Rh28) was established from an alveolar rhabdomyosarcoma. Rh28 displays a constant population doubling time of 45-55 h until passage 60, when the doubling time progressively increases until proliferation ceases. Loss of proliferative capacity is associated with morphological evidence of differentiation to multinucleated myotubes, fusion, and the expression of numerous muscle-specific genes. In contrast to normal myogenic differentiation, multinucleated cells continue to synthesize DNA and express abundant c-myc transcripts. These observations suggest synchronous replication and possible arrest in the G2-phase of the cell cycle, since there was no evidence of mitotic activity in differentiated cells. Terminal differentiation of early passage Rh28 cells was induced in the presence of 10% dialyzed fetal calf serum but not by medium containing 2% undialyzed serum, suggesting a role for low molecular weight growth factors in this process. Our data indicate that the Rh28 cell line may be of value in elucidating the relationship between oncogenic transformation and differentiation in rhabdomyosarcoma.

Biochemical mechanisms in colon xenografts: thymidylate synthase as a target for therapy.

Growth of human adenocarcinomas of the colon and rectum in immunoincompetent mice has allowed for a greater understanding of the interaction of 5-fluorouracil, its metabolism, and mechanism(s) of cytotoxicity under conditions of tumor growth in situ. Conversely, this agent has proven to be a useful tool in defining metabolic characteristics in human colon adenocarcinomas. Analysis of tumor sensitivity to 5-fluorouracil (FUra),5-fluorouridine (FUrd) and 5-fluoro-2'-deoxyuridine (FdUrd) suggests that growth inhibition in vivo is related to a DNA-directed event. Resistance, de novo appears to be a consequence of relatively transient inhibition of the target enzyme thymidylate synthase (dTMP-synthase), which may be a consequence of low concentrations of 5,10-methylenetetrahydrofolate (CH2-H4PteGlu) or its polyglutamate forms within tumor cells in situ. In order to study the relationship between inhibition of dTMP-synthase and growth inhibition, mutant cells deficient in their ability to salvage dThd have been selected, and grown as xenografts. Data suggest that transient inhibition of dTMP-synthase and not dThd salvage is responsible for resistance de novo, and that prolonged inhibition of dTMP-synthase would be a lethal event in vivo. This would predict that a cell lacking dTMP-synthase activity would not be tumorigenic. This has been tested directly by selecting clones of GC3 colon adenocarcinoma cells deficient in dTMP-synthase (TS-) activity. Preliminary data indicate that each of 3 TS- clones is tumorigenic in athymic nude mice. The importance of dTMP-synthase as a target for drug development is discussed with respect to these findings.